In known processes of electrostatic imaging, a image may be recorded in the form of an electrostatic latent image upon a photoconductive member. A developed image may then be obtained from the latent image by application of toner particles, which may be in the form of a finely powdered solid or a liquid dispersion.
In the electrostatic imaging process employing liquid toner, the basic steps are :
1. Generating an electrostatic latent image, e.g. on a photoconductive surface such as a plate or drum which has been sensitized by charging with a corona discharge and by exposure to light in the form of an image of an object-- the charge is dissipated in exposed areas; PA0 2. Developing the latent image by contact with a liquid dispersion ("toner") of fine particles which in a positive process are attracted to the charged areas and which in a reverse process are attracted to the uncharged areas; PA0 3. Removal of excess liquid toner and particles from the background areas; PA0 4. Transfer of the image, e.g. to a substrate such as paper, under influence of an electrical field; PA0 5. Fixing the image by fusing the particles together and to the substrate; and PA0 6. Cleaning the plate or drum for reuse.
In this connection reference is made to the following issued U.S. patents in the field of electrophotography: U.S. Pat. Nos. 3,990,696, 4,233,381, 4,253,656, 4,256,820, 4,269,504, 4,278,884, 4,286,039, 4,302,093, 4,326,644, 4,326,792, 4,334,762, 4,350,333, 4,355,883, 4,362,297, 4,364,460, 4,364,657, 4,364,661, 4,368,881, 4,378,422, 4,392,742, 4,396,187, 4,400,079, 4,411,976 4,412,383, 4,413,048, 4,418,903, 4,420,244, 4,435,068, 4,439,035, 4,454,215, 4,460,667, 4,473,865, 4,480,825, 4,501,486, 4,522,484, 4,531,824, 4,538,899, 4,582,774, 4,585,329, 4,586,810, 4,589,761, 4,598,992, 4,603,766, 4,620,699, 4,627,705 and 4,678,317, the disclosures of all of which are incorporated by reference herein.
Following development, the excess liquid toner and particles are removed from the image, by a step often referred to as "metering". This step may be carried out by use of an electrically biased reverse roller which also shears off the "fluffy" part of the imaged regions. At the transfer stage, there is likely to occur a degree of smudging, smearing or squashing of the image, which detracts from its quality.
In published UK patent application GB 2169416A (Landa et al), the disclosure of which is incorporated herein by reference, a degree of squash-resistance is imparted to the developed image by using fibrous toner particles which may be prepared as follows: a thermoplastic polymer (and pigment) is plasticized with a nonpolar liquid, preferably at elevated temperature, to form a sponge, the sponge is shredded, more nonpolar liquid is added, the pieces of shredded sponge are wetground into particles and the grinding is continued to pull the particles apart to form fibers extending therefrom (the particles preferably having a diameter less than 5 microns). Finally, a charge director may be added to impart a charge of predetermined polarity to the toner particles.
In an alternative method of preparation which is also the subject of published UK patent application GB 2176904A (Landa et al), the disclosure of which is incorporated herein by reference, a thermoplastic polymer is plasticized at an elevated temperature with a nonpolar liquid, a pigment is stirred into the plasticized polymer to disperse it therein, stirring is continued to prevent the formation of a sponge while reducing viscosity of the mixture by adding additional nonpolar liquid to form a dispersion, cooling while continuing stirring to permit the precipitation of pigmented polymer (toner) particles having a plurality of fibers and withdrawing the dispersion having a concentration of toner particles from the mixing step. Charge director may be added as before.
It is self-evident that the principal difference between the content of the toner image formed with dry toner and that formed with liquid toner, is that the toner image formed with liquid toner contains the solvent, usually a hydrocarbon solvent, in which the toner particles are suspended, this solvent being absent from the image formed using dry toner. In order to produce a visible image when using a liquid toner, which resembles the image obtained when using a solid toner, the prior art which relates to fusing and fixing a developed image by the application of heat thereto assumes the simultaneous removal by evaporation of the toner solvent.
Thus, in U.S. Pat. No. 4,474,456 (Kobayashi et al), which relates to a heat-fixing device including a heat plate along which transfer medium bearing thereon a toner image is advanced during which the toner image becomes fixed to the transfer medium by fusing, it is stated that in an electrophotographic copying machine of the wet-development type, a sufficient amount of thermal energy must be applied to the transfer medium to remove the carrier liquid impregnated into the transfer medium by evaporation.
Published European Patent Application 0247248A1 discloses a process in which an electrostatic charge pattern is developed with liquid toner and the developed image is fixed by the simultaneous application of heat and pressure, characterized by the fact that the toner particles have a particular melt viscosity in the dry state. The disclosure contemplates evaporation during fixing of a substantial part of the carrier liquid, removal of the vapors thus produced by suction or by pressurized entrainer gas, and disposal of the removed vapors by adsorption or absorption, condensation or combustion.
Published European Patent Application 0244198A2 describes and claims a reproducing machine (and a corresponding method) including apparatus for applying heat to sheet material having a liquid-developed image thereon to remove substantially all of the liquid carrier transferred thereto so as to dry the sheet of support material and at least partially melt the pigmented particles transferred thereto, as well as apparatus for generating heat to the pigmented particles so as to fuse them to the support in image configuration.
U.S. Pat. No. 4,727,394 (Bov et al) claims a reproducing machine incorporating a hot fusing roll cooperating with a pressure roll to apply heat and pressure to a developed image on a sheet of support material so as to vaporize substantially of the liquid carrier transferred thereto and to substantially permanently fuse the pigmented particles to the sheet of support material.
The disadvantage of these prior art methods is that removal of organic solvent from the developed image as recited therein creates a pollution problem in the immediate environment, which must be countered, if pollution is to be avoided, by taking special measures as set out in above-mentioned published European Patent Application 0247248A1, namely by adsorption or absorption, condensation or combustion.